The present invention relates to a novel nozzle hole arrangement for a gaseous and liquid fuel injection valve with concentric needles for an internal combustion engine.
The United States Environmental Protection Agency proposes that by the year 2004, it will require heavy-duty diesel engines to reduce their oxides of nitrogen (NOx) emissions by 50% from the legislated 1998 4.0 g/bhp.hr to 2.0 g/bhp.hr and with no increase over the 1998 particulate matter legislated level set at 0.1 g/bhp.hr. There is also pressure to reduce carbon dioxide emissions from engines to reduce the global greenhouse effect. However, diesel engines have high thermal efficiency and have an advantage of emitting very low levels of hydrocarbons and carbon monoxide. Accordingly, diesel engines are expected to remain the dominant heavy-duty transport power plant.
Engine manufacturers are researching ways to reduce diesel engine emissions while maintaining performance, power, and efficiency. For example, manufacturers are researching methods adapting diesel engines to use natural gas as fuel in substitution for diesel fuel. Compared to diesel fuel, natural gas is a cleaner burning fuel. The main constituent of natural gas is methane, which, with respect to diesel, has higher heating value, lower adiabatic flame temperature and simpler molecular structure. Compared to the refining processes employed to produce petroleum products, such as diesel fuel, less processing is required to prepare natural gas for consumption in an internal combustion engine. An additional benefit of using natural gas is that it is abundantly available on all continents. Other gaseous fuels, such as propane and hydrogen are also cleaner burning compared to diesel fuel and may also be substituted for diesel fuel to yield reduced emissions. Accordingly, it will be understood that references herein to xe2x80x9cgaseous fuelxe2x80x9d refer to all such cleaner burning fuels.
A number of different methods are being researched to allow gaseous fuels to be substituted for diesel fuel in compression ignition engines. For example, some natural gas fuelled engines use a method known as xe2x80x9cfumigationxe2x80x9d wherein the natural gas is introduced into the combustion chamber with the intake air. Such engines may use a spark plug or an injection of pilot fuel to initiate combustion. A problem with fumigation is that to avoid engine knock, a compromise must be made at high loads by limiting at least one of engine compression ratio or fuel quantity. Another compromise is typically required at low loads, in that the intake air may need to be throttled to ensure that the fuel to air ratio is sufficient to support combustion. These compromises result in a reduced thermal efficiency and reduced torque at low speed, compared to a comparable conventional diesel-fuelled engine.
Another method of substituting gaseous fuel for diesel fuel in a compression ignition engine employs High Pressure Direct Injection (HPDI) of the gaseous fuel. In this method, gaseous fuel at approximately 200 bar is injected directly into the engine combustion chamber at the end of the compression process. A few crank angle degrees earlier, a small amount of pilot fuel is injected into the engine combustion chamber. The pilot fuel is a fuel that auto-ignites more readily than the gaseous fuel. For example, the pilot fuel may be diesel fuel or dimethylether (DME). The pilot fuel autoignites and then ignites the gaseous fuel. The engine can keep the same compression ratio as its original diesel counterpart because there is no knock limitation. In addition, there is no need for throttling. The thermal efficiency is thereby maintained.
One approach to implementing HPDI in a compression ignition engine employs at least two separate fuel injection valves, requiring at least two mounting ports in the cylinder head. This approach has additional development costs associated with designing and manufacturing a new cylinder head, adding significantly to the cost of making new engines or retrofitting existing diesel-fuelled engines. Costs and modifications necessary to convert a conventional diesel engine to an HPDI engine can be reduced by employing a gaseous and liquid fuel injection valve that fits into the same mounting port as a conventional diesel injection valve. Conventional diesel engines and HPDI engines may then share the same cylinder head.
A gaseous and liquid fuel injection valve controls the separate and sequential injection of the pilot fuel and gaseous fuel into the engine cylinder through a different series of holes in separate concentric nozzle tips. Such an injection valve is disclosed in U.S. Pat. No. 6,073,862 (xe2x80x9cthe ""862 patentxe2x80x9d), which is incorporated herein by reference in its entirety. In preferred designs of the fuel injection valve, the pilot fuel injection valve body is the inner valve assembly and the gaseous fuel injection valve is concentrically arranged in the annular space around the pilot fuel injection valve. The nozzles of early injection valves comprised six equally spaced pilot fuel holes and six equally spaced gaseous fuel holes. One aspect of the preferred arrangement is that the pilot fuel injection valve body is free to rotate around its longitudinal axis. Compared to an assembly that is not allowed to rotate, a freely rotating inner needle valve assembly is a simpler design that is easier and less expensive to make. For large bore engines with many cylinders, a freely rotating valve assembly is more desirable compared to a needle with its holes in a fixed orientation because it is preferable to be able to tune each engine cylinder individually. That is, the same hole orientation may not be optimized for each and every cylinder.
A problem observed with HPDI engines employing concentric gaseous and liquid fuel injection valves is that when the engine operates at medium and light loads there can be a periodic variation in engine speed and NOx emissions. The amplitude of these variations can be particularly large when the load is small. These unstable conditions can lead to uncontrollable overspeeding and halting (that is, engine stalling). Accordingly, there is a need to prevent such unstable conditions. The same problem was not observed when the same engine was run with conventional fuel injection valves and using only diesel fuel. Various aspects of the engine design were studied to determine the cause for the unstable conditions, including the design of a gaseous and liquid fuel injection valve.
An improved gaseous and liquid fuel injection valve comprises:
(a) an injection valve body;
(b) a pilot fuel needle valve assembly disposed within the body, the pilot fuel needle valve assembly comprising a first needle movable in the direction of the longitudinal axis of the body between an open and a closed position for controlling injection of pilot fuel into a combustion chamber of the engine through a plurality of pilot fuel holes formed in a nozzle tip of the pilot needle valve assembly;
(c) a gaseous fuel needle valve assembly disposed within the body and concentric with the pilot fuel needle valve assembly, the gaseous fuel needle valve assembly comprising a second needle movable in the direction of the longitudinal axis between an open and a closed position for controlling the injection of gaseous fuel into the combustion chamber through a plurality of gaseous fuel holes formed in a nozzle tip of the gaseous fuel needle valve assembly.
At least one of the nozzle tips is rotatable to thereby change the position of the pilot fuel holes with respect to the gaseous fuel holes, and the number of the plurality of pilot fuel holes is different from the number of the plurality of gaseous fuel holes.
Preferably, the plurality of gaseous fuel injection holes number N and the plurality of pilot fuel injection holes number M, where N and M do not have any common factors other than one. Additionally, it is preferable for at least one of N or M to be a prime number. To improve the contact between the burning pilot fuel or the burned products and the gaseous fuel jets, it is preferable for N and M to be about the same in number, without being equal in number. Preferably the absolute value of Nxe2x88x92M is equal to one, two, three or four, such as, for example, when N is 7 and M is 6, or when N is 8 and M is 7.
Another consideration in determining the preferred number of pilot fuel holes and gaseous fuel holes is the statistically calculated variance from the average value of the angle between the centerlines of the gaseous fuel holes and pilot fuel holes. Preferably, this variance is less than about 0.4 degree2, and more preferably, less than about 0.2 degree2.
The pilot fuel holes and the gaseous fuel holes are preferably equally spaced around the circumference of the respective nozzle tips. In a preferred embodiment, the gaseous fuel needle valve assembly occupies an annular space formed between the body and the pilot fuel needle valve assembly. In this embodiment, the pilot fuel needle valve assembly may preferably comprise an outer housing that is movable within the body and operates as the second needle for the gaseous fuel needle valve assembly.
To control the sequential injection of pilot fuel and gaseous fuel, the pilot fuel needle valve assembly and the gaseous fuel needle valve assembly are preferably independently operable. Such an arrangement allows pilot fuel and gaseous fuel to be injected at different times, directly from the respective valve assemblies into the combustion chamber.
In a preferred arrangement the first needle is movable to the open position by moving in an inward direction away from the nozzle tip and the combustion chamber. Similarly, the second needle is preferably movable to the open position by moving in an inward direction away from the nozzle tip and the combustion chamber.